Wireless communication systems based on the well-known Universal Mobile Telecommunication System (UMTS) will eventually evolve to a packet data system with an orthogonal frequency division multiple access (OFDMA) air interface. In this evolution, reverse link transmissions of data packets are scheduled. Scheduling reverse link transmissions involves allocating or assigning a set of one or more sub-carriers to a mobile station at every scheduling instance. The sub-carriers allocated to a particular mobile station will be localized in the sense that the allocated sub-carriers are contiguous.
A drawback with localized sub-carrier allocation is that the reverse link transmission is spread over a narrow bandwidth and not spread over a wide bandwidth. Spreading the reverse link transmission over a narrow bandwidth does not allow the reverse link transmission to experience much, if any, frequency diversity. For example, suppose the sub-carriers are modulated onto a carrier with a 5 MHz bandwidth and each sub-carrier has a 15 kHz bandwidth. If a mobile station is allocated 12 contiguous sub-carriers, then that mobile station would only be using a small or narrow portion, i.e., 180 kHz (15 kHz×12), of the 5 MHz carrier bandwidth.
To achieve frequency diversity without de-localizing sub-carrier allocation, frequency hopping is employed for re-transmissions of data packets. An initial data packet transmission will occur over a first set of contiguous sub-carriers. Data packet re-transmissions will occur over different sets of contiguous sub-carriers in the frequency domain thereby allowing frequency diversity to be achieved over re-transmissions.
Frequency hopping may be implemented in a number of ways. One manner of implementing frequency hopping is via a scheduler at a base station or some other entity. In this implementation, the scheduler makes a scheduling decision which determines the sub-carriers to which the mobile station should hop and use for its re-transmission. A separate scheduling decision is made for each re-transmission and in a manner compatible with prior scheduling decisions and/or restrictions. Upon making each scheduling decision, information indicating the locations (or identities) of the sub-carriers being allocated for the re-transmission (also referred to herein as “hopping locations”) is signaled from the base station (or other entity in which the scheduler resides) to the mobile station. Signaling the hopping locations each time a re-transmission is necessary increases signaling overhead, which is undesirable because it reduces system capacity.
Another manner of implementing frequency hopping is to use predetermined frequency hopping patterns in which the mobile station will know to hop to a predetermined set of sub-carriers (or location) for its re-transmissions, e.g., mobile station will hop 10 sub-carriers every time it re-transmits a data packet. However, in this implementation, the scheduler does not have control of all scheduling decisions. Scheduling decisions with respect to reverse link re-transmissions are made by the mobile stations and may be incompatible with restrictions and/or scheduling decisions made by the scheduler or other mobile stations.
Accordingly, there exists a need for implementing frequency hopping without sacrificing scheduling control while reducing signaling overhead.